Synergies

For each skill on this website it says what skills it adds what value to.

By update 2.6.0 skills has synergies. They takes your existing skills and provides a buff to other skills within the same skill tree, typically within the same element.

So if you have for example Double Claw, synergies always looks like this: 

• Overhead Triple Claw +18 damage
• Vampiric Overhead Claw +4 damage
• Flame Spin +8 damage/s
• Frost Bite +8 damage
• Venom +2 damage/s
  

then it is only multiplied according to the level of the original skill, here Double Claw.
It means if we choose Venom that gives +2 by every level of Double Claw.

Double Claw 1 -> +2 damage/s Venom
Double Claw 2 -> +4 damage/s Venom
Double Claw 3 -> +6 damage/s Venom
Double Claw 4 -> +8 damage/s Venom
Double Claw 5 -> +10 damage/s Venom
Double Claw 6 -> +12 damage/s Venom
Double Claw 7 -> +14 damage/s Venom
Double Claw 8 -> +16 damage/s Venom
Double Claw 9 -> +18 damage/s Venom
Double Claw 10 -> +20 damage/s Venom
Double Claw 11 -> +22 damage/s Venom
Double Claw 12 -> +24 damage/s Venom
Double Claw 13 -> +26 damage/s Venom